Actuators with rotary ratchet advances are used to accurately position the trim of control or injection valves known as "choke valves" in the oil field industry. Choke valves are used to restrict or stop the flow of process fluids and are typically designed to handle highly abrasive fluids. A choke valve placed on a well head, for example, may be used to control the flow of well bore fluids in cooperation with a gate valve that is used as a safety shut-off valve which can rapidly stop the flow of the well bore fluids. A choke valve is often used in such applications to minimize, the flowrate of the highly abrasive well bore fluids before the gate valve is closed and exposed to the fluids, because at full flow the abrasive fluids might damage the sealing capabilities of the gate valve.
In a typical subsea wellhead installation, the choke valve is replaceable subsea, but the gate valve can only be replaced by retrieving the wellhead to the surface. Thus, it is important to protect the gate valve during emergency shutdown operations so to avoid retrieving the wellhead to repair any damage.
Conventional choke valve ratchet actuators require about ten seconds per pressure pulse to rotate the actuator output shaft one increment, which is approximately one-eighth of a six pitch revolution. Thus, with a two inch travel valve stem, a choke valve actuator can take as long as sixteen minutes to move the valve trim between the fully opened and the fully closed positions. With ratchet actuators, if you loose electrical power, hydraulic power, or the electrical position feedback device fails, you can count the hydraulic pulses to get to a known position. Conventional gate valves, on the other hand, can move between fully opened and closed positions in as little as ten seconds. As a result, a gate valve must often be used in emergency shut-down conditions to stop the flow of well bore fluids before the choke valve can minimize the flowrate of the well bore fluids. This results in possible damage to the gate valve and the need to retrieve the subsea wellhead.
It is known in the industry to use continuous-operation actuators with choke valves. These actuators can move the valve between fully opened and closed positions in approximately thirty seconds. However, these continuous-operation actuators cannot accurately position the trim of the choke valve to desired settings between the opened and closed positions. Accurate trim settings are required, for example, when the pressure drop across the choke valve is very high, because small variations in the valve trim can result in large changes in the flowrate of the well bore fluids.